An insulated gate bipolar transistor (IGBT) is a type of semiconductor power switching element that has been used to an increasing extent for a relatively long time in power electronics, since it combines features of a bipolar transistor (good on-state behavior, high reverse voltage) and features of a field-effect transistor (FET) (virtually power-free driving). A certain amount of robustness to short circuits is also obtained, since the IGBT limits load current. IGBT power switches are often used in the high-power range since they typically have a high reverse voltage of, for example, 6.6 kV, and may switch high currents, for example up to 3 kA.
An IGBT power switch typically involves intelligent driving and reliable operating point monitoring to realize the full performance potential and to simultaneously ensure reliable switching operation. The driver circuit for driving the IGBT should be able to switch the IGBT on and off without destruction in each operating state defined in a data sheet, for example, destruction being possible during a bridge or load short circuit.
It is generally known that an IGBT can be protected from states which are dangerous for the IGBT, for example an overcurrent or short circuit, using desaturation monitoring and a two-stage switching-off operation.
In this case, the collector-emitter voltage Vce, which is an indicator of overloading, is measured during the switching-on phase of the IGBT. If an overcurrent occurs, the collector-emitter voltage Vce rises above a value Vcesat which is known, for example, from a data sheet for the IGBT. The rise can be detected by a comparator. However, due to varying switching-on inertia of different IGBTs, the comparator is not activated until after a particular interval of time, i.e., there is a delay between the switching-on moment of the IGBT and the activation of the comparator.
In order to be able to use a driver circuit for desaturation monitoring for a plurality of different types of IGBTs, the delay and the voltage threshold of the collector-emitter voltage Vce are set separately from one another in order to make optimum use of the respective type of IGBT.
However, this means that setting the delay and the collector-emitter voltage Vce independently of one another would involve using two pins of a package at which the parameters can be set independently of one another with the aid of external voltages and corresponding components. These additional two pins are not readily available in modern standardized 16-pin IC packages if yet further monitoring functions which can be set are intended to be implemented in the driver circuit. Even if packages having a sufficiently large number of free pins are available, the manner in which the delay and collector-emitter voltage are set results in an increase in the complexity and costs of the IGBT.
Furthermore, desaturation monitoring is already used in bipolar power transistors in order to protect against an overcurrent and/or short-circuit current, but a plurality of drive modules are also used.
In addition, it is advisable to switch off the IGBT in a “soft” manner as it were, that is to say in a plurality of stages, since, in the event of overloading or a short circuit, the IGBT may be destroyed by the production of a large collector-emitter overvoltage during the rapid switching-off operation. The operation of switching off an IGBT or a metal-oxide-semiconductor FET (MOSFET) in a plurality of stages is known from more modern drivers, for example the TD350 driver marketed by STMicroeletronics.
In this case, the voltage threshold of a first switching-off stage and an interval of time, in which the voltage threshold is kept at a particular voltage level, are used to switch off the IGBT in a “soft” manner, the values for the voltage threshold and the corresponding interval of time, in which the voltage threshold is kept constant, depending to a very significant extent on the design of the power inverter and varying for different types of IGBTs.
In this case, the voltage threshold and the interval of time are usually also set using two separate pins, which is again associated with increased complexity and higher costs since the standardized 16-pin IC packages cannot be readily used.